Mössbauer studies on athermal martensite formation in an fe-ni-mn alloy

In this study, austenite-martensite phase transformations which are formed by cooling effect in Fe-30% Ni-0.2% Mn alloy are investigated with Mössbauer spectroscopy and scanning electron microscopy. The single peak of the paramagnetic phase and the six peaks of the ferromagnetic phase of Fe-30% Ni-0.2% Mn alloy were observed in the Mössbauer spectrum. The internal magnetic field strength of ferromagnetic martensite phase was determined as 33.8T and the isomer shift values were determined as - 0.11 mm.secsu?1 and - 0.06 mm.secsu?1, respectively, for the austenite and martensite phases. In this alloy, athermal transformation was observed. The results obtained are in agreement with literature.     

预变形对FeNiC合金马氏体相变形态影响

研究了母相预变形对Fe-23Ni-0.55C合金马氏体相变表面浮凸形貌、组织形态的影响.结果表明:母相预拉伸变形和压缩变形对Fe-23Ni-0.55C合金马氏体影响有所不同.母相预拉伸变形,其马氏体由透镜状变为细针状,表面浮凸角增大;母相预压缩变形,随变形量的增大,不但其马氏体边缘破碎,而且马氏体中脊发生弯曲、甚至断裂.但是母相预变形并未改变其马氏体相变的惯习面和表面浮凸形貌.惯习面仍为{259}f,表面浮凸形貌均为"N"型.     

形变时效对FeMnSiCrNi合金形状回复率和相变温度的影响

为了进一步提高Fe-14Mn-6Si-8Cr-5Ni合金的形状记忆效应,对固溶态合金采用了形变时效的方法处理,并利用光学显微分析、X射线衍射分析和透射电子显微分析的测试手段分析了时效温度和时效时间对合金形状回复率和相变温度的影响.结果表明,固溶态合金经10%拉伸和600℃时效10 min时,形状回复率提高幅度最大,由固溶态的48%提高到84.7%,并且合金γ→ε马氏体转变的起始温度Ms由固溶态的34℃降低到13.2℃.合金的形状回复率得到提高的主要原因是合金中热诱发ε马氏体已经消失,组织为奥氏体和大量定向α’马氏体,这样的组织特征有利于应力诱发γ→ε马氏体相变以及它们的逆相变.     

新型含铝奥氏体耐热合金Fe-20Cr-30Ni-0.6Nb-2Al-Mo的动态再结晶行为

在Gleeble-3500热力模拟试验机上对一种新型奥氏体耐热合金(Fe-20Cr-30Ni-0.6Nb-2Al-Mo)进行单道次热压缩实验,结合OM、EBSD及TEM等表征手段,研究了该合金在950~1 100℃和0.01~1s-1热变形参数下的动态再结晶行为,采用回归法确定了合金的热变形激活能和表观应力指数,并以此构建其高温本构模型。实验结果表明,新型奥氏体耐热合金的应力水平随变形温度的升高而降低,随应变速率的增大而升高;动态再结晶行为更易发生在较高变形温度或较低应变速率下。采用lnθ-ε曲线的三次多项式拟合求解临界再结晶拐点的方法,较准确地预测了合金的动态再结晶临界点。此外,归纳出该合金在动态再结晶过程中的形核机制,主要包括应变诱导晶界迁移、晶粒碎化以及亚晶的合并。     

Quantitative microstructural characterisation of Fe–30Ni alloy after martensitic transformations by means of stereological and magnetic methods

In the Fe–30Ni alloy investigated a martensitic transformation can occur both during quenching or plastic deformation. Martensite formed during plastic deformation, depending on the thermo-mechanical treatment applied, exhibits a different morphology from that achieved during quenching and forms the so-called composite-like structure. The morphology and volume fraction of martensite depends both on strain and temperature. In the present studies Fe–30Ni alloy was deformed by monotonic rolling in one path and perpendicular rolling in the temperature range M_D–M_S. The aim of the investigations was a determination of martensite volume fraction depending on the strain and temperature. To examine the influence of strain, the alloy was deformed by rolling in one path or perpendicular rolling at a temperature of − 30 °C, in the strain range of 10–30%. The dependence of temperature was investigated by rolling with 30% strain in a temperature range from − 30 °C to − 80 °C. The variants of thermo-mechanical treatment performed enabled us to achieve different martensite morphologies and volume fractions. Microstructural analysis was performed by means of light microscopy and transmission electron microscopy. The results of quantitative microstructural analysis of martensite and retained austenite volume fractions formed in different thermo-mechanical treatments were compared with those obtained by magnetic measurements. The fraction of deformation-induced martensite determined varied from 2% to 86%. The partial volume fractions V_V^MF of martensite formed in different deformation directions were also determined. It was found that the influence of the temperature on the martensite volume fraction is more pronounced than the influence of strain.     

Size distribution of martensite plates in an Fe-Ni-Mn alloy

In this paper, the size distribution of the martensite plates in an Fe-23.2 Ni-2.81 Mn (wt%) alloy, which transforms isothermally at subzero temperatures, is reported. The distribution of the martensite plates has been determined as a function of the reaction temperature, volume fraction of martensite, the austenitic grain size, a superimposed elastic stress and prior plastic strain (at room temperature) of austenite. Increasing the driving force either by decreasing the reaction temperature or by a superimposed elastic stress changes the size distribution by enhancing the extent of radial growth of the martensite plates. Pre-straining of austenite does not allow the martensite plates to grow to the full extent. The present results show that the radial growth of the martensite plates increases with increasing driving force and decreases due to work-hardening of austenite. The transformation is found to progress through a combination of the spreading-out of clusters and filling-in of pockets, both occurring simultaneously. However, the extent of filling-in, i.e. compartmentalization of austenite grains, is more in the coarse-grained (0.09 mm) and medium-grained (0.048 mm) specimens compared to that in the fine-grained (0.019 mm) specimens.     

Cu-3.0Ni-0.64Si合金的热变形行为

对Cu-3.0Ni-0.64Si合金进行了变形温度为750~900℃、变形速率为0.001~1s~(-1)条件下的等温压缩实验。结果表明,随着变形温度升高或变形速率降低,峰值应力明显降低,合金容易发生动态再结晶。通过线性回归分析,求得Cu-3.0Ni-0.64Si合金的变形激活能为410.4kJ/mol,建立了Cu-3.0Ni-0.64Si合金的高温热变形流变应力本构方程6)ε=e~(40.56)[sinh(0.017σ)]~(5.21)exp[-410.4×10~3/(RT)]。分别讨论了变形温度和变形速率对Cu-3.0Ni-0.64Si合金在等温压缩变形中显微组织的影响。最后基于动态材料模型理论,用Prasad失稳判据,得到不同真应变量下的热加工图。优化后的工艺参数为变形温度860~900℃和变形速率0.002~0.01s~(-1)。     

The effect of deformation on γ-ε martensitic transformation in an Fe-Mn-Mo alloy

Effect of prior plastic deformation of austenite on the martensite start temperature, volume fraction and strength of martensite have been studied in an Fe-14.3%Mn-3.7%Mo alloy. Mo was chosen to examine the possible effect of the third alloying element in an Fe-Mn based alloy and the obtained results were compared with those of the Fe-Mn binary alloys given in the literature. Predeformation of austenite created considerable changes on the formation characteristics and also the strength of the martensitic phase and the obtained results were discussed in terms of the dislocations formed during the deformation process.     

Cryogenic stability of retained austenite in Fe–Cr–Ni weld metals obtained by laser welding

Fe–Cr–Ni alloy is the most potential substitute for Ni-based alloys as consumable in low-temperature nickel steel welding. In this study, six groups of Fe–Cr–Ni weld metals with different chemical composition were fabricated by single-pass laser welding. The volume fraction of retained austenite (RA) in the weld metals in as-welded condition increased from 0 to 30.2% with the increment of amounts of alloy elements (Cr, Ni and Mn). The thermal stability of RA was investigated by deep cryogenic treatment (DCT) to see whether sufficient RA can be maintained at low temperature. The results revealed that if the initial content of RA was?<??~?18%, RA would not transform into martensite after DCT. The surrounding martensite can hinder the transformation of RA, which plays a dominant role in the cryogenic stability of RA. The higher the strength/hardness of the surrounding martensite, the stronger the resistance to the transformation of RA. The carbon content of the surrounding martensite is the crucial factor affecting its strength/hardness. However, if the alloy elements amounts were too large, the thermal stability of RA would decrease and some of RA would transform to martensite after DCT. The newly formed martensite (fresh martensite) increased strain concentration of the weld metal, which enhanced the strain energy of martensite transformation and therefore restrained the further transformation of RA. The content of RA in the Fe-13.497Cr-7.249Ni-0.93Mn and Fe-15.548Cr-7.622Ni-0.961Mn weld metals after DCT dropped to?~?18%. The optimum of initial content of RA in the weld metals for low-temperature toughness is?~?18%.

     

Cu-Ni-Si合金冷变形及动态再结晶行为研究

研究了时效温度和时效时间对不同冷变形条件下Cu-2.0Ni-0.5Si合金性能的影响。在Gleeble-1500D热模拟试验机上,采用高温等温压缩试验,对Cu-2.0Ni-0.5Si合金在高温压缩变形中的流变应力行为进行了研究。结果表明,合金经900℃固溶,当变形量为40%,时效温度达到450℃时,其显微硬度达到201HV,导电率达到34%IACS。随变形温度升高,合金的流变应力下降,随应变速率提高,流变应力增大。在应变温度为700、800℃时,合金热压缩变形流变应力出现了明显的峰值应力,表现为连续动态再结晶特征。从流变应力、应变速率和温度的相关性,得出了该合金高温热压缩变形时的变形激活能Q。     

Microstructure and abrasive wear resistance of an alloyed ductile iron subjected to deep cryogenic and austempering treatments

To further improve the mechanical performance of a new alloyed austempered ductile iron(ADI), deep cryogenic treatment(DCT) has been adopted to investigate the effect of DCT time on the microstructure and mechanical behaviors of the alloyed ADI Fe-3.55 C-1.97 Si-3.79 Ni-0.71 Cu-0.92 Mo-0.64 Cr-0.36 Mn-0.30 V(in wt.%). With increasing the DCT time, more austenite transformed to martensite and very fine carbides precipitated in martensite in the extended period of DCT. The amount of austenite decreased in alloyed ductile irons, while that of martensite and carbide precipitation increased. The alloyed ADI after DCT for 6 h had the highest hardness and compressive strength, which can be attributed to the formation of more plate-like martensite and the finely precipitated carbides. There was a gradual decrease in hardness and compressive strength with increasing the DCT time to 12 h because of the dissolution of M3 C carbide. After tempering, there was a decrease in mechanical properties compared to the direct DCT sample, which was caused by the occurrence of Ostwald ripening of precipitated carbides. The optimum wear resistance was achieved for the alloyed ADI after DCT for 6 h. The wear mechanism of the alloyed ADI in associating with DCT is mainly consisted of micro-cutting wear and some plastic deformation wear.     

钨含量对新型高铬锰氮双相不锈钢Cr29Mn12Ni2N0.6Wx组织和性能的影响

研究了钨含量对新型高铬锰氮双相不锈钢Cr29Mn12Ni2N0.6Wx(x=1,2,3)显微组织、力学性能和耐腐蚀性能的影响。结果表明:Cr29Mn12Ni2N0.6Wx不锈钢固溶处理后具有典型的铁素体+奥氏体双相组织,铁素体含量在45%~60%范围内;随着钨含量的增加,合金中σ相的析出倾向增强,铁素体含量增加,合金的耐腐蚀性能降低,屈服强度和抗拉强度升高;经1 050℃固溶处理30 min后,该系列双相不锈钢中不再有σ相析出,其屈服强度大于650 MPa,抗拉强度大于900 MPa,断后伸长率大于30%,作为高强度资源节约型超级双相不锈钢具有潜在应用前景。     

A Comparative Study on Crystal Structure and Magnetic Properties of Fe-Mn-Si and Fe-Mn-Si-Cr Alloys

In this study, the crystal structure and magnetic properties of Fe-30Mn-6Si (wt.%) and Fe-30Mn-6Si-5Cr (wt.%) alloys were compared by using X-Ray Diffraction (XRD) and Physical Properties Measurement System (PPMS) measurements. Detailed analysis of diffractograms at room temperature demonstrates that the Cr-free sample contains austenite and martensite phases, but for Cr-added sample the martensite phase disappears. According to micro hardness measurements, the presence of chromium decreased the hardness of the alloy. The magnetic saturation values at room temperature were measured as 11.32 emu/g for Fe-30Mn-6Si (wt.%) alloy and 18.34 emu/g for Fe-30Mn-6Si-5Cr (wt.%) alloy. The addition of Cr increased the magnetic saturation value of FeMnSi alloy while for both systems the hysteresis loop was quite narrow. As a result, both alloys exhibited soft magnetic characteristic.     

Influence of α′ martensite on shape memory effect in Fe-14Mn-5Si-9Cr-5Ni alloy

Abstract

The strain induced γ → ε and γ → α′ martensite transformations were examined by X-ray diffraction in an Fe-14Mn-5Si-9Cr-5Ni alloy. From the original microstructures of single phase austenite and two phase austenite and α′ martensite, the alloy was examined after prestraining at room temperature and recovery heating at 673 K, and finally, its shape memory effect was determined. It was shown that the alloy with the original two phase microstructure of autensite and α′ martensite exhibits a higher degree of shape recovery. The presence of α′ martensite in the original microstructure before prestraining can considerably improve the extent of shape recovery.     

Very high strain-rate response of a NiTi shape-memory alloy

The compressive response of a NiTi shape-memory alloy is investigated at high strain rates, using UCSD’s modified split Hopkinson pressure bar and a mini-Hopkinson bar with specially designed striker bars. To obtain a constant strain rate during the formation of the stress-induced martensite phase in a Hopkinson test, a copper-tube pulse shaper of suitable dimensions or a stepped striker bar is employed, since without a pulse shaper or with a uniform striker bar, the strain rate of the sample will vary significantly as the material’s microstructure changes from austenite to martensite, whereas with proper pulse shaping techniques a nearly constant strain rate can be achieved over a certain deformation range. At a very high strain rate, the yield stress and the stress-induced martensite formation process are significantly different from those at moderately high strain rates, suggesting that, correspondingly, different microstructural changes may be involved in the phase transition regime. The material’s yield stress appears lower when measured in a mini-Hopkinson bar (with very small samples) as compared with that measured by a 1/2-in. Hopkinson bar (with relatively large samples), possibly due to the sample size that may produce different deformation mechanisms within the superelastic strain range. The transition stress from the austenite to the martensite phase shows strain-rate sensitivity. This may be explained by considering the interfacial motion of the formed martensite phase, based on the thermally activated and dislocation-drag models. There exists a certain critical strain-rate level, at which the transition stress for the stress-induced martensite formation equals the yield stress of the austenite phase. Therefore, the shape-memory alloy deforms by the formation of stress-induced martensites, accompanied by the yielding of the martensite phase at this critical strain rate, while the material deforms plastically by the dislocation-induced plastic slip at strain rates above this critical level.     

30CrNi3MoV低合金超高强钢中的马氏体相变

通过对马氏体的显微组织进行分析,并结合线膨胀试验得到的相变动力学信息研究了30CrNi3MoV低合金超高强钢中的马氏体相变特征.结果表明:淬火冷却30CrNi3MoV钢的相变产物包括低碳板条状和高碳针状两种马氏体形态,两者的形成在动力学曲线中截然分开.板条马氏体形成于Ms以下的较高温(310℃～260℃),相变过程中发生了碳的重新分配,造成富碳奥氏体微区的形成;高碳针状马氏体形成于Ms以下的较低温(260℃～170℃),由富碳奥氏体微区转变而成.板条马氏体形成速率远高于针状马氏体.     

Fe-36Ni高温高应变率动态力学性能及其本构关系

为研究Fe-36Ni因瓦合金的动态力学性能及其本构关系,在20～800℃和10-3～104 s-1的应变率内,采用电子万能试验机和高温分离式霍普金森压杆分别对Fe-36Ni因瓦合金进行准静态实验和动态压缩实验,得到其高温、高应变率下的应力-应变曲线.结果表明,Fe-36Ni因瓦合金的流动应力表现出较强的应变率和温度敏感性,随着应变率的增大而增大,随着温度的升高而减小.采用改进应变率项和温度项的Johnson-Cook本构方程拟合了Fe-36Ni因瓦合金在高温、高应变率下的动态塑性本构关系,拟合结果与试验数据吻合很好.     

Ti6213合金的热加工图研究

在Gleeble-1500热模拟机上进行了Ti6213合金热模拟压缩试验,变形温度范围为800-1050℃,应变速率范围为0.001-10 s-1,最大变形量为60%,并根据动态材料模型建立了加工图。结果表明,合金在高温变形时主要有2个合适的加工区域,一个是变形温度800-950℃,应变速率0.01 s-1以下区域;另一个在相变温度以下40℃内,应变速率10 s-1以上区域。在900-930℃和0.001 s-1的变形条件下,出现耗散率峰值为65%,高m值,S形应力和应变速率对数曲线的现象,合金表现出超塑特性。拉伸实验进一步表明,延伸率可达512%,组织为两相混合组织。另外,合金在800-930℃和大于0.01 s-1的条件下出现集中变形带,表现为局部流变特征。     

Maraging Behavior in an Fe-19.5Ni-5Mn Alloy I: Precipitation Characteristics

The aging behavior of an Fe-19.5Ni-5Mn alloy has been studied in detail. A substantial maraging-hardening response was obtained upon aging at between 300–550°C, and it displayed classical hardening behavior. The pronounced hardness was attributed to strain hardening caused by coherent, fine spherical precipitates. The activation energy for precipitation, calculated from microhardness data, was 41 kcal/gmole. Ordered fct θ-NiMn precipitates were identified with two different shapes, depending on the aging temperature. Higher aging temperatures resulted in disk-shaped precipitates, while rod precipitates appeared at lower temperatures. Twin-related Widmanstätten austenite grains appeared in a lenticular shape that were coupled together by a twin boundary. Their orientation relationship with the parent martensite was found to be of the Kurdjumov-Sachs (K-S) type.     

The morphology of martensite in Fe-C,Fe-Ni-C and Fe-Cr-C alloys

The morphology of martensite in widely varying series of Fe-C, Fe-Ni-C and Fe-Cr-C alloys was investigated using optical microscopy. The effects of formation temperature and alloying elements on the martensite morphology were studied in detail. It was found that in Fe-C alloys, lath martensite forms in alloys with less than 0.8wt% carbon, butterfly martensite forms in alloys with between 0.98 and 1.42wt% carbon and lenticular martensite forms in alloys with more than 1.56wt% carbon. In Fe-Ni-C alloys, four different martensite morphologies form depending upon the formation temperature and composition, and for alloys of a fixed carbon content the martensite morphology changes from lath to butterfly to lenticular to thin plate as the formation temperature is decreased. In Fe-Cr-C alloys, lath martensite forms at high temperature, and below the lath formation temperature mainly {2 2 5}_f plate martensite is formed. Based on the results obtained, the importance of the strength of austenite, and the austenite stacking fault energy to the martensite morphology was discussed.